The growing presence of U.N. monitors already has helped the situation in Syria, opposition activists said, though it hasn't completely stopped the bloodshed that has been a fact of life in the Middle Eastern nation for more than 13 months.

"Shelling has calmed, but this does not mean that the (Kofi Annan-brokered) peace plan has been implemented," said an activist in Homs identified as Saleem, who reported at least three killed Monday in that central Syrian city. "Gunfire, rocket shelling, mortar shelling and arbitrary arrests still occur."

Saleem credited U.N. monitors in Homs, a hot spot of the opposition movement and frequent target of government forces, with helping retrieve bodies left in the streets, in some cases for as long as 50 days. And in Hama, a Local Coordination Committees of Syria (LCC) member named Mousab said the situation in his city is "more quiet," while adding that civilians are not able to talk to U.N. observers because they are always flanked by government forces.

But Ahmad, another opposition activist in Idlib province in northwestern Syria, offered a sour assessment of the U.N. effort. Observers presence may stifle violence in the short-term, he said, but things can change quickly after they depart.

"Unfortunately, the monitors are like a guide for the Syrian regime: Wherever they go, usually people are killed after they leave," Ahmad said Monday night.

Some of the worst violence Monday occurred in Idlib, the capital of Ahmad's province.

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Twin car bomb blasts there left at least 20 people, said the Syrian Observatory for Human Rights. The state-run Syrian Arab News Agency (SANA), meanwhile, reported eight civilian and law enforcement members were killed.

A third explosion hours later didn't produce any more casualties, according to the observatory.

State media reported that about 100 people -- mostly civilians -- were injured in the first set of blasts. Citing medical sources, the observatory said the dead were mostly security force members. One car bomb targeted a government security building, said the LCC.

Photos, posted on SANA's website, showed buildings in ruins, cars crushed by debris and the bloody remains of victims.

U.N. observers visited the sites of the deadly car bomb blasts, state media reported. U.N. Secretary-General Ban Ki-moon later Monday issued a statement condemning what he called "terrorist bomb attacks" in Idlib as well as the capital of Damascus.

Opposition activists and state media both reported explosions and gunfire early Monday in the latter city, though they traded accusations as to who was responsible.

Syrian state-run TV showed images Monday from the capital of a damaged police car and a pillar with a missing chunk of concrete outside the Central Bank of Syria building.

The report claimed both were targeted by an "armed terrorist group" using rocket-propelled grenades, adding that four police officers were wounded in the first attack and no casualties were reported from the bank attack.

But the LCC claimed the government was responsible and "has resorted to fabricating staged explosions that have taken the lives of dozens of Syrians," characterizing recent blasts in or near government buildings as "suspicious."

"These tricks no longer fool anyone, especially given the fact that the regime has resorted to these escalations every time there is political movement at the Arab, regional or international level to find a political solution to the crisis in which the regime kills its people who are demanding freedom," the opposition network said.

Not including the Idlib bomb blasts, the LCC reported 11 dead Monday around Syria -- including three apiece in Homs and Idlib.

The same group reported that the death toll before dawn on Tuesday was already nearly double that. Fifteen of the 20 fatalities -- including six members of one family -- that were counted as of 5:30 a.m. were in villages in greater Idlib province.

CNN cannot independently verify reports of violence and deaths within Syria, as the government has restricted access by most of the international media.

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Syria has been engulfed in violence since March 2011, when government forces started cracking down on demonstrators who were peacefully protesting al-Assad's regime. The president's family has ruled Syria for 42 years. Some opposition members, including a number of military defectors, have since taken up arms against the regime forces.

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The United Nations estimates that at least 9,000 people have died in the conflict, while opposition groups put the death toll at more than 11,000.

Both the Syrian regime and the rebel Free Syrian Army have accepted a peace plan laid out by Annan, the U.N.-Arab League special envoy. A key element of the plan involves a cease-fire by all parties, plus the withdrawal of Syrian forces from populated areas.

Another component is the presence of unarmed U.N. military observers. About 30 of them were on the ground Monday, and a total of 50 are expected by Friday, said Annan spokesman Ahmad Fawzi via Twitter. The U.N. Security Council has authorized up to 300 monitors in Syria.

"This is a matter of utmost urgency for the United Nations, and all efforts are in place to make sure that we get the people on the ground as quickly as possible," Neeraj Singh, the observer team's spokesman, said Sunday.

Since the cease-fire deadline passed April 12, more than 700 people have been killed, the LCC reports.

The U.N. monitoring mission's leader Maj. Gen. Robert Mood, who arrived in Damascus on Sunday, conceded that his group's efforts are futile unless all sides commit to peace.

"Ten unarmed observers, 30 unarmed observers, 300 unarmed observers, even 1,000 unarmed observers cannot solve all the problems," Mood told reporters. "So I call on everyone to help us and cooperate with us in this very challenging task ahead of us."